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We were here

In the early hours of a Monday morning in July 1945, the world’s first atomic bomb test lit up a remote corner of New Mexico. Several weeks later, two more atomic bombs were dropped on large urban centres in Japan. These events marked the beginning of the ‘atomic age,’ but they also marked another beginning, a brief pulse that an experimental biologist would call Time Zero.

Since my post on the ‘natural’ genetics experiment on rescue workers at the World Trade Center site, I’ve been thinking more about the unintentionally brilliant experiments that can emerge from disasters and accidents. One of the most remarkable examples is the so-called bomb-pulse, which is the global isotopic signature left by the atomic bomb tests of the 1950s and 1960s. That signature is found in every living thing on the planet and can now be read back like a ticking clock. It can tell us the birth year of an unidentified murder victim, a vintage wine, your brain cells, fat cells or even the molecules of fat themselves. But it also left an enduring message for future scientists in the geological record. The message says: We were here.

Dog 2 – 19kt, Nevada Test Site, May1951. Image Public Domain, with many thanks to Trinity Atomic Web Site. Click on the photo to visit this fascinating archive of historical documents and media on nuclear weapons.

Before 1945, the global levels of naturally-occurring radioactive isotopes were steady. Between 1955 and 1963, an intense period of cold war-fueled nuclear weapons development caused a sudden increase in the levels of certain isotopes in the atmosphere. This increase came to a sharp end with the signing of the 1963 Partial Test Ban Treaty (PTBT), an agreement that dramatically reduced the number and power of such tests. Bomb pulse testing makes use of the spike in atmospheric levels of the harmless isotope carbon-14 (14C), which doubled between 1945 and 1963. 14C is normally produced at a low rate by the action of cosmic rays in the upper atmosphere, but 99% of the carbon on earth is in the non-radioactive 12C form. The ratio between 14C and 12C levels in living things reflects that of the atmosphere. Plants take up the 14C and 12C in the form of carbon dioxide, convert it into sugars, are eaten by animals, who in turn may be eaten by other animals.

We can estimate the ‘birth date’ of molecules within a living thing because the levels of 14C have been decreasing at a steady rate since 1963. This regular decrease is due to the gradual dissipation of the isotope into the ocean and into living things, as well as dilution due to the burning of fossil fuels (which are rich in 12C). By comparing the historical records of atmospheric 14C ratios to the ratios in say, a vintage bottle of Australian red, we can determine the year in which the grapes were grown. Similarly, the 14C ratio of the tooth enamel of an unidentified body can tell us their year of birth to an accuracy of less than two years. We can do this because the enamel is only formed at very specific times in childhood.

But most of our bodies are not made of permanent structures like tooth enamel. We are each a colony of different kinds of cells that are constantly growing, dying, and renewing. The bomb pulse allows us to measure the birth and lifespans of these different kinds of cells, giving us an average ‘age’ for the different cells of our body. To do this, we measure the 14C ratio of the DNA molecules in each cell, since DNA is made only at the time the cell is first formed (during cell division). Many of the discoveries made using this technique have settled acrimonious debates or overturned long-held models. For instance, it showed that the neurons of your neocortex (the ‘brainiest’ bit of our brain) have the same birthday as you do. In other words, you’re stuck with the neurons you were born with (you can read a summary at Not Exactly Rocket Science.) Another (of many) high profile findings of the same group was that you are not stuck with the fat cells you are born with – most fat cells die and are replaced by a new cell about once a decade. Last month, a new study was published that looked at the molecules of fat within those fat cells, and found that their average age was about 1.6 years. They also found that the average age of fat molecules in obese people is about 50% higher than in non-obese people, probably because the rate of fat removal is slower.

As interesting and useful as all these methods are, we are probably only going to be able to use them for another generation or two, since atmospheric 14C levels should be back to their pre-cold war levels by about 2020. However, there will also be a more long-lived legacy of the bomb pulse: the sudden spike of isotopes in the geological record. The sediments being laid down today will contain organic matter with higher levels of 14C. Will this become a distinct ‘event boundary’ like the iridium-rich K-T boundary that records the arrival of an asteroid and the extinction of (many) dinosaurs? Geologists are currently arguing about that possibility as part of the wider debate about whether to formally recognise a new geological epoch – the Anthropocene. Informally, the Anthropocene designates the modern age, under the hypothesis that human activity has changed the planet as profoundly as many other major geological events. Some geologists argue that the bomb pulse would be the best candidate for the official stratigraphic boundary of the Anthropocene. It’s unambiguous, global, and sharp.

To have a debate about how geologists in the future should classify the evidence of our existence is a charmingly human activity. I look forward to the squabble continuing for some decades. But whatever the outcome of the debate, no matter how long our civilization lasts, whether it flares magnesium bright or fades into the darkness, it will be much, much longer before all traces of our existence are gone. We were definitely here.